A. Field of Invention
This invention relates to an improved electrowinning anode and its method of manufacture. The anode is a lead alloy sheet joined to a copper busbar. A pin is inserted through the joint between the sheet and the busbar and a layer of lead is electrodeposited over the busbar, pin, and the joint between the busbar and the anode sheet.
B. State of the Art
Lead alloys have been used for many years as electrowinning anodes for the recovery of copper, nickel, and zinc from sulfate solutions. For most uses, the anode is produced by casting lead into a mold to produce the anode size and shape. The lead or lead alloy flows around a copper busbar to provide electrical contact between the anode sheet and the copper busbar. The lead may also serve as a protective barrier to prevent attack of the copper busbar from acid mist or direct electrolyte impingement.
Rolled lead alloy sheet for anodes offers advantages of reduced porosity, more uniform cross section, more uniform grain structure, and reduced corrosion rates than cast anodes. The rolled lead alloy sheet, however, must be joined to the copper busbar as shown in FIG. 1. In most cases the lead anode sheet (1) is joined to the copper busbar (2) by first casting lead around the busbar (3) as shown in FIG. 1 and subsequently welding or burning the rolled sheet to the lead cast around the busbar. The lead cast around the busbar may be the same composition as the anode sheet or a different alloy may be used to cast around the busbar prior to attachment of the sheet by welding or burning.
A new method to attach the rolled lead alloy sheet to a copper busbar was developed in U.S. Pat. No. 4,373,654. In this system a rolled lead-calcium-tin alloy sheet is joined to a copper busbar by means of a solder joint in a slot in the busbar. This method gives a complete metallurgical bond between the sheet and bar unlike that attained in the cast around the bar system. A thin coating of lead-tin alloy was formed on the bar by dipping prior to the soldering process to protect the copper busbar from acid mist or direct impingement of electrolyte. However, this did not completely protect the busbar, and in use some attack of the copper busbar occurred. Eventually the solder joint was exposed and attacked.
The attack on the busbar was prevented by an improvement developed in U.S. Patent No. 5,172,850. As shown in FIG. 2, in this improvement the rolled lead alloy sheet (4) is joined to the copper busbar (5) by means of a soldered joint (6) in slot (7) of the busbar. A thin coating of lead tin alloy (8) is formed on the surface of the copper busbar by dipping prior to the soldering process. The soldered joint is sealed by puddling a filler alloy (9) into all crevices. A coating of lead (10) is electrodeposited onto the surface of the busbar and over the joint between the busbar and the anode sheet to give a complete metallurgical seal around the busbar and joint. The finished anode of this method provides excellent resistance to attack by the acid mist or direct impingement of electrolyte onto the busbar area.
A lead anode sheet soldered into the slot of a copper busbar and subsequently coated with a layer of electrodeposited lead onto the busbar and over the joint between the busbar and the anode sheet produces an anode with a complete metallurgical bond between anode sheet and busbar as well as a complete seal around the busbar and joint. This method of producing electrowinning anodes has been proved to produce low resistance between busbar and anode sheet while protecting the busbar and joint from attack from acid mists or direct impingement of electrolyte onto the bar and soldered joint.
In some cases premature failure of the anodes has occurred by damage to the soldered joint at the edge of the sheet. This damage is caused by dropping the anode from some distance such that the busbar contacts the side of the cell and is bent upward. This deformation of the busbar can cause delamination of the bar from the anode sheet at one end of the soldered joint. Breaking of the solder bond can provide entry of electrolyte into the joint causing corrosion of the busbar/anode sheet interface.
The anode may be accidentally lifted and subsequently dropped during pulling of the cathodes when the crane accidentally picks up an anode as well as the cathode. Since the hook is not long enough to hold the anode, it falls back into the tank as the cathodes are lifted from the cell. Other incidents such as short circuits where dendrites grow from the copper cathode and attach themselves to the anode result in raising the anode part way out of the cell during pulling the cathode. When the bond between anode and cathode is broken due to the weight of the anode, the anode falls back into the cell.
The anode may also be damaged when the anodes are removed from the cell to remove adhering deposits or flakes or to clean the sludge from the cell. In such cases the anodes are removed from the cell by a crane and placed on racks. Depending on the experience of the crane operator the anodes may be dropped onto the racks. The anodes may also be handled mechanically such as to remove the PbO.sub.2 /MnO.sub.2 anode deposit as in zinc electrowinning. Such handling may include dropping the anode which may damage the edge of the soldered joint. The force of the falling anode contacting the side of the cell can result in bending of the busbar. When the busbar is bent most of the force is transmitted to the edge of the busbar/anode sheet interface because this interface is more rigid than the busbar alone. If the force is sufficient, the busbar may be peeled away from the anode sheet at the end of the solder joint.
The present invention provides a method of protecting the busbar/anode sheet soldered joint from damage due to mishandling.